to add to what 009jim said above: Gearing is all about ratios. Lets say your front ring is 36t and your cog is 36t. Ratio is 1:1 meaning one rev of the pedal is one rev of the wheel. Let's say you have a 50t ring and a 10t cog. Ratio is 5:1. One turn of the pedal equal 5 turns of the wheels. This takes a lot more force to turn.